Field of the Invention
The present invention relates to a method of repairing turbine blades.
The method of repair is primarily but not exclusively directed to the type of turbine blade in which adjacent blades are interconnected by lacing wires, clamping wires, cover bands or other members which require a hole or other discontinuity to be formed in at least some of the turbine blades, such hole or discontinuity hereinafter being referred to for convenience as a discontinuity.
The purpose of lacing wires, cover bands etc. is to prevent excessive relative movement between the blade and to dampen any vibration during operation of the turbine.
During use of the turbine, inevitably the blades are subjected to wear and damage and particularly in the case of steam turbines they are subjected to the effect known as stress corrosion cracking which is particularly prevalent at the low pressure end of the turbine where it is fairly common for lacing wires to be present in turbine blade arrays.
Whereas it has been the practice for many years to replace damaged turbine blades, new methods of repair now enable satisfactory repairs to be carried out to damaged blades, some of which methods of repair may be carried out while the blades are still in situ on the rotor, thereby making a very significant decrease in the time normally taken to effect such repairs and minimising possible damage to the blades that may well occur if they have to be separated from the rotor assembly.
It has been found by the applicants that irrespective of whether or not the blade is repaired whilst on the rotor, discontinuities are a considerable problem. Whereas it may appear that a satisfactory repair can be carried out to correct damage to the blade, the intense heat necessary during a welding operation leads to a concentration of stress around a discontinuity such as a lacing wire hole, which concentration of stress could later manifest itself in the form of cracks appearing around the lacing wire hole after the blade is put back into use.
It is also necessary in some cases to subject a turbine blade, preferably before repair, to a physical force to bend or pre-distort or otherwise deform the blade with the intention of returning it to its proper position, or displacing it slightly from its proper position with the knowledge that the repair step to be carried out is likely to result in a blade at the end of the repair taking up its proper position or being as close thereto as possible.
It is sometimes necessary to apply heat to the blade to enable such predistortion, this is essential when the blade is of substantial thickness.
It may be that in some cases the repair to be carried out is only at the leading outer edge of the turbine blade and hence it is only necessary to fill, by welding in a plug or filling with weld metal, the lacing wire hole in the vicinity of the weld area.
If however a physical force has to be applied to the blade, as mentioned above, then the whole of the blade may be effected by said physical force and the existence of other lacing wire holes, even though they are not in the vicinity of the weld area, may cause a concentration of stess around the lacing wire holes and, in extreme circumstances the application of physical force can cause distortion of the lacing wire hole making it difficult to replace the lacing wires.
Any input of stress to the blade would necessitate a heat treatment process which itself can lead to concentration of stress around discontinuities.
To overcome this problem, applicants have proposed that such discontinuities are always filled with weld metal prior to the commencement of any welding operation. This seemingly unnecessary step leads to a vast improvement in the reliability of the repaired blades.
As the methods of repair become more intricate and apparatus is developed for repairing turbine blades while in situ on the rotor, it has been found that whereas damage to leading and trailing edges may be repaired whilst the blades are in situ on the rotor, the filling of discontinuities with weld metal may not in all cases be possible since either access to the blade by welding apparatus in the position where the discontinuity is situated is impossible or, even though filling of the discontinuity might be possible, subsequent redrilling of the hole after the repair has been carried out is not possible.
Since applicants are aware that an absence to fill the discontinuity is unsatisfactory, it did not therefore appear to be possible to repair such blades whilst still on the rotor and be confident of a satisfactory service life after repair.